The invention relates to a method and composition for densifying aluminum additives, by compaction of aluminum (Al) and iron (Fe) fines into a physical form such as a briquette, thereby reducing aluminum losses to the tap slag and improving residual aluminum control during primary deoxidation of liquid steel in the ladle.
Generally, molten steel resulting from converting a combination of molten iron and scrap steel in a basic oxygen furnace or electric arc furnace has to be deoxidized prior to its solidification in ingots or in continuously cast shapes. Such deoxidation occurs mainly in the steel ladle during tapping from the furnace or somewhat later at the ladle metallurgy station. Deoxidation is for the purpose of reducing the dissolved oxygen content of the molten steel to a predetermined and measurable narrow range required by the ultimate quality of the steel product. This implies the addition to the molten steel of specified amounts of deoxidizing agents such as carbon, manganese, silicon and aluminum generally used in combination. In addition to their deoxidizing function, these same elements may be added also for the purpose of forming an alloy with the steel to thereby alter the physical and mechanical properties of the latter. Several other, more expensive elements added to molten steel for alloying purposes, such as boron, vanadium, niobium, titanium and calcium, may also offer deoxidizing power but they are generally protected from being wasted to that function by introduction only after the dissolved oxygen has been reduced by at least two (2) of the elements cited, C, Mn, Si and Al.
Until the introduction of continuous casting and the almost compete conversion from ingot casting to xe2x80x9cstrandxe2x80x9d casting around 1985, oxygen was one of the allies of the steelmaker. By controlling the carbon/oxygen balance, using little deoxidation except the specified carbon and manganese and occasionally some silicon and very little aluminum during ingot teeming, steelmakers produced well over 75% of all steel world-wide as good quality xe2x80x9crimmingxe2x80x9d steel ingots. The carbon monoxide (CO) gas developed by the carbon/oxygen reaction during solidification was xe2x80x9crimmed outxe2x80x9d leaving no porosity in the clean, virtually inclusion-free surface layers of these ingots, while most internal porosities trapped by the final solidification were sealed without trace by the hot rolling process.
Unfortunately, rimming is not applicable to continuous casting because the high speed of solidification overtakes the upward flow of CO bubbles, entrapping large amounts of porosity in subsurface layers of as-cast steel. Today, over 95% of all steel is continuously cast, and it is xe2x80x9ckilledxe2x80x9d as opposed to xe2x80x9clivexe2x80x9d or rimming. As the terms are used in this application, xe2x80x9ckilledxe2x80x9d or deoxidized state means that the amount of xe2x80x9cfreexe2x80x9d or dissolved oxygen contents have dropped from 150-400 ppm oxygen typical of rimmed steels down to 20-50 ppm oxygen in silicon and manganese killed (SiK) xe2x80x9clongxe2x80x9d steel products representing some 40% of total steel production. Free oxygen is further reduced down to the 0.1-5 ppm range in aluminum (Al) killed or AK xe2x80x9cflat-rolledxe2x80x9d products, about 60% of all steels. From the one-to-two order of magnitude difference in free oxygen resulting from aluminum and manganese (Al/Mn) as compared to silicon and manganese (Si/Mn) deoxidation, it is clear that Al/Mn vastly outperforms Si/Mn for full deoxidation and total absence of CO porosity.
However, even with superior deoxidation power, aluminum is not used on all strand cast steels because the products of aluminum deoxidation are alumina and aluminate spinel inclusions, which are solid at the steelmaking temperature of 3000xc2x0 F. It is known that solid inclusions passing through the narrow tundish-to-mold metering nozzles tend to clog these refractory nozzles and to shut down the whole casting operation. This is particularly prevalent with the smaller nozzles used for billet, bloom and xe2x80x9cdogbonexe2x80x9d casting sections of the long products group of which only a small fraction, the Special Bar Quality (SBQ) subgroup may use a little aluminum xe2x80x9csacrificially,xe2x80x9d typically about 1 lb/ton to prevent porosity while avoiding nozzle blockage. With the very large, oversized nozzles used for the large sections of slab casting of flat steel products, the alumina build-up problem is not sufficiently severe to prevent full aluminum deoxidation. Thus, flat product steels are virtually all aluminum deoxidized or Aluminum Killed (AK) using between 2 and 7 lb aluminum per ton of steel. The minimum amount of aluminum needed to deoxidize molten steel tapped from the furnace can be estimated as follows: the dissolved oxygen content of the steel before deoxidation varies typically from 600 to 1200 ppm. The deoxidation reaction is 2 Al+3/2 O2=Al2O3. Converting this into approximate weights: 2xc3x9727+3xc3x9716=102. This means that, for the reaction to be completed, 48 weight units of oxygen require 54 weight units of aluminum. Thus, 600 ppm O require 675 ppm Al, and 1200 ppm O require 1350 ppm Al. As 500 ppm equal one pound per ton, the minimum amount of aluminum required to deoxidize steel is 1.35 to 2.7 lb per ton. On the one hand, manganese and carbon take some share of the deoxidation work which reduces the need for Al by 0.5 to 1 lb per ton. On the other hand, most steels require a retained or residual Al content of 0.025 to 0.050% corresponding to an additional xc2xd to 1 lb per ton. In short, the minimum amount of aluminum required for the large group of low carbon AK steels is about 2 lbs per ton if no parasitic losses are encountered.
When fully deoxidizing with aluminum at tap, the average usage of aluminum is approximately 5 lb/ton, of which it is believed, without being held to any one particular theory or mode of operation, about 3 lb/ton are lost to slag (90%) and air (10%). Thus 2.7-2.9 lb aluminum per ton are lost to parasitic reactions with the slag involuntarily transferred with the molten steel tapping from the furnace.
Both Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) slags result from oxidizing carbon (C), Silicon (Si), manganese (Mn), and even some iron (Fe) to purify hot metal (BOF) and scrap (EAF), to achieve steel specifications and to raise the temperature to 3,000xc2x0 F. required at tap. Only carbon leaves the melt as carbon monoxide (CO) gas. Some of it may stay in the slag but only as entrapped bubbles. The other elements form oxides, e.g., silicon dioxide (SiO2), manganese oxide (MnO), iron oxide (FeO), etc., which are fluxed to a liquid phase by the proper amount of burnt lime/dololime (CaO+MgO) and form a fluid slag which is poured in a slag pot after the steel is tapped out.
Typical chemical compositions of this furnace slag from a normal BOF or EAF melting and oxidizing process are as follows (in weight percent).
Cpd. GaO SiO2 MgO Al2O3 FeO MnO P2O5 Cr2O3 TiO2 Na2O K2O
Wt.% 45 15 12 1.5 18 6 1.5 0.3 0.4 0.2 0.1
Typical aluminum consumption for such process is 4 lb/ton (loss 2 lb/ton).
Typical chemical compositions of this furnace slag from a highly energized EAF feeding a modern thin slab casting plant, are as follows (in weight percent).
Cpd. CaO SiO2 MgO Al2O3 FeO MnO P2O5 Cr2O3 TiO2 Na2O K2O
Wt.% 27 11 9 1.0 45 5 1.0 0.6 0.3 0.1
Typical aluminum consumption for such process is 6.5 lb/ton (loss 4.5 lb/ton).
The percentage of FeO is the major difference between the two slags, directly causing the difference in aluminum losses. The parasitic reaction, reducing the FeO of the slag by aluminum is strongly exothermic, thus resulting in the following:
3 FeO (slag)+2 Al (deox)xe2x86x92Al2O3 (new slag component)+3 Fe (new steel)
Similar, but less exothermic reactions occur with MnO, SiO2, P2O5, Cr2O3, TiO2 and the alkalis and all contribute to aluminum losses. However, the reaction between Al and FeO represents 60-90% of all of the aluminum losses during tap, depending on the relative FeO content as illustrated by the slag chemistry.
One of the first efforts at reducing deoxidation aluminum loss involved reducing the parasitic oxidation of the aluminum by air, using for example, argon or nitrogen gas shrouding the entire ladle. This effort was essentially unsuccessful. It was only when the focus centered on the slag that aluminum losses started to decrease. Involuntarily carried over from the Basic Oxygen Furnace (BOF) or Electric Arc Furnace (EAF), the slag covers the steel in the ladle since the very first seconds of the tapping process that lasts from 2-12 minutes. Unless special devices are in place, the very first liquid to hit the bottom of the ladle is not steel, but slag, setting the stage for high aluminum losses and other chemistry control problems. The largest part of the volume of slag carried over comes toward the end of the tap, when steel and slag are entrained together through the tap hole in a vortexing manner. Since about 1990, the focus has been to prevent the parasitic (FeO+MnO) reaction, and polluting (SiO2+P2O5+Cr2O3. . . ) reactions forming oxides, from furnace slag transfer to the ladle, has received serious attention from both steel producers and suppliers of slag retention devices. Competition between various designs such as balls, pyramids, or darts and more sophisticated systems (EBT, photocells) has resulted in reducing the volume of slag transfer by as much as 80-90%.
Benefits have included aluminum savings on the order of 0.5-1 lb/ton, and also improved manganese recovery and iron yield. The main advantage with today""s tightening steel chemistry specifications, are much improved control of aluminum and reduced xe2x80x9cpick-upxe2x80x9d of pollutants Si, P, Cr, Ti, Cr, . . . by the liquid steel. However, aluminum savings from furnace carry-over slag control, while contributing to justify the cost of the devices, comes short of the potential 2-4.5 lb/ton to be saved toward the theoretical minimum.
Traditionally, essentially pure metallic aluminum has been the most common form of aluminum deoxidizing agent used in the steelmaking process, in any convenient form such as notch bars, small ingots, shot and chopped wire. The use of essentially pure aluminum presents some significant disadvantages, however, arising primarily from its low density as compared to the molten steel to which the aluminum is added. The density of liquid aluminum at steelmaking temperatures of approximately 1600xc2x0 C. is only about 2 grams (g) per cubic centimeter (cc), whereas the density of molten steel at the same temperature is greater than 7 g/cc. Therefore, when aluminum is added to the melt, it will float at the steel/slag interface, where the aluminum rapidly oxidizes, with relatively small amounts of the aluminum actually making contact with the molten steel. The efficiency of the aluminum as a deoxidizing agent is thus limited by the rate at which oxygen in the melt can diffuse upward to the slag/steel interface, and deoxidation performance is erratic.
The more successful approaches toward deox Al savings came from the suppliers and can be summarized as reducing or eliminating contact between deox Al and ladle slag. For the purpose of this invention, these approaches can be considered more or less specifically as Prior Art.
The first group, called xe2x80x9cplungersxe2x80x9d or xe2x80x9cdunkersxe2x80x9d consists of hanging from the small hoist of the ladle crane a scrap steel bloom holding at its tip either a canister full of deox Al (Pierce cage, 1965-85) or a cast ring of deox Al (MSSI, 1985-95), to fit around the bottom of the bloom. After the tap, the ladle full of steel with slag on top, the bloom is lowered at once and quickly forced through the slag layer. Al starts melting and dissolving in the liquid steel only when it is at least a foot below the slag-metal line. Despite large Al savings of the order of 1.5 to 2.5 lb/ton and much better Al chemistry control, steel producing management gave up on the dunkers which were very unpopular with melters because of the additional work and attention to detail required.
The evolutive breakthrough from this group was solid aluminum wire, xc2xd in diameter, feeding through the ladle slag, which is not applicable to tapping conditions but is the most accepted form of deoxidation aluminum for delayed aluminum deoxidation and chemistry adjustment at the Ladle Metallurgical Furnace (LMF). Only nominally more expensive than ingot-type deoxidation aluminum, the vertical downward pushing of a xc2xd wire of solid aluminum from a continuous coil accomplishes the submergence below the slag of the ladle plungers and yields very high recoveries on the order of 90% and better. In addition, it allows simple and precise metering of the addition needed to the final aluminum content required by the specifications. Other alloying elements, C, S, Nb, V, Ti, B, . . . are also xe2x80x9cwired inxe2x80x9d at the LMF today for high yield and precision of chemistry.
Another approach introduced by the Japanese refractory producers, consisted of pushing the slag layer toward the side of the ladle during and after tap by strong argon bubbling through a plug in the ladle bottom and of maintaining the steel meniscus clean of slag by lowering a refractory ring floating on the steel to keep the slag out. All the ferroalloy and aluminum additions are then made with much improved recoveries. Briefly tried in the United States, it was abandoned due to the complexity of implementation.
Ferroaluminum has also been used as a deoxidation additive as a 35-38% aluminum/65-62% iron alloy, the only narrow range on the entire Fexe2x80x94Al binary diagram suitable for use a deoxidation aluminum. This range supplies the desired combination of stability (no decrepitation below 40 and above 75% aluminum), density (about 2.5 times that of pure aluminum and not cost-effective to be on the Al-rich side of the binary diagram), and resistance to oxidation (below 34% aluminum, the alloy rusts like regular iron). Produced in large induction melting furnaces from aluminum scrap and low carbon steel scrap, the alloy is both moldable into popular deoxidation aluminum shapes and crushable to suitable sizes. Instead of the typical 4.5 lb/ton needed with regular deoxidation aluminum ingots at 95-99% aluminum, only 2-2.5 lb contained aluminum per ton are needed with 35% ferroaluminum which translates into 5.7-7.1 lb of the alloy per ton of liquid steel tapped. The density of ferroaluminum is about twice the density of pure aluminum, resulting in deeper penetration of the ferroaluminum oxidizing agent into the molten steel. Because of the deeper penetration, resulting in improved contact between the deoxidizing agents and the molten steel, ferroaluminum does produce an improved deoxidation efficiency in comparison to aluminum alone but still suffers from certain disadvantages. The melting and casting costs of the alloy became such that 35% aluminum cost almost as much per pound as 98% aluminum. The aluminum units saved were insufficient to cover the additional cost of aluminum in the alloy. Still used on a limited scale, mostly for adjustments at the LMF instead of wire, the 35% aluminum ferroalloy has been abandoned as deoxidation aluminum replacement at tap for AK steels.
Another technique to densify deoxidation aluminum is taught by U.S. Pat. No. 4,801,328 and consists mainly of casting molten secondary aluminum around precut reinforcing steel bars in notch bar molds. The major advantage of this invention over the ferroaluminum alloy was the reduced cost of production, requiring only, at the secondary aluminum remelting plant, to insert precut rebar steel pieces into the notch bar molds prior to casting the liquid aluminum. Another advantage is the flexibility in selecting the desired aluminum to iron ratio for best performance which is reported to be about equal to that of the ferroaluminum alloy, but at lower aluminum unit cost. It was invented more specifically for the long products group to improve control of the sacrificial aluminum addition prior to Si/Mn deoxidation at tap. It has not however, penetrated the much larger flat-rolled AK steel market for deoxidation aluminum. A major cause of this failure is the 0.4% carbon, 0.2% silicon of rebar steel contributing highly undesirable impurities for this very large low carbon, low Si group of steels. This approach runs against the current trend toward less remelting of aluminum scrap into deoxidation aluminum ingots due to mounting energy costs. It is far more cost-effective to directly compact the suitably sized scrap into briquettes and other solids such as xe2x80x9chockey pucksxe2x80x9d used extensively by large aluminum scrap generators.
Another approach to steel deoxidation involved low-pressure, binder consolidated large one pound subspherical blends of coarse aluminum chips and highly oxidized ferromanganese fine rejects from ferromanganese producing blast furnaces, using between 8-10% molasses as a binder. This technology suffered from several problems, including the fact that the quality of the fines was so low and variable, and in general too oxidized, that an excessive fraction of the aluminum was consumed in reducing the manganese fines first, resulting in excellent and very predictable manganese recoveries, but less than desirable aluminum yields. Aluminum, being the expensive ingredient, made the overall economics borderline. Optimal aluminum/manganese tap alloying needs also varied from facility to facility and from grade to grade, making it difficult to adjust one composition to these varying needs. A specific blend was required for almost every group of steel grades. At least one publication, attributed the well-known black pencil line defect, very common at that time on the surface of cold-rolled low carbon steel sheets, to manganese fines in contact with deoxidizing aluminum at tap.
Because of the increasing awareness for the need to control and eliminate these impurities in steel, a great deal of activity has been focused on developing systems or techniques that fill this need, none of which have been completely successful. The art teaches that previous generations of metallurgists and steelmakers have been unable to solve the deoxidation aluminum recovery issue at tap and extremely low and inefficient tap aluminum recoveries are ranging from as low at 15% in ultra-low carbon steels, high FeO slags (up to 55%) of the flat roll mini-mills to more acceptable 35-40% yields of controlled carry-over slag volumes and low FeO tap slags in some Basic Oxygen Furnace (BOF) shops. The need still exists for consistent improvement from these indicative numbers, each in their particular environment.
The invention involves a simple solution to improving aluminum efficiency in the severe environment of the steel producing furnace tapping in the ladle without requiring additional work from the furnace crews.
It is an object of this invention to submerge the deoxidizing aluminum additive in briquette or other compressed form below the molten slag and into the molten steel by increasing its apparent density in order to reduce or preclude its parasitic reaction with the highly oxidizing ladle slag.
It is another object of this invention to provide an aluminum-based deoxidizing agent which does not involve any cost-prohibitive melting.
It is another object of this invention to increase the density of this recycled aluminum scrap or secondary aluminum of a size approximately 4 mesh by down by physical blending with low carbon steel scrap, seconds, off-spec powders or any other unalloyed, recyclable iron (Fe) units all about 4 mesh approximately within the same size range as the aluminum, prior to the conventional briquetting or other shape compacting process such as pressed cylinders known in the trade as xe2x80x9chockey pucks.xe2x80x9d As used in this invention, the terms briquetting or other shape compacting process are used interchangeably.
It is still another object of this invention to increase the apparent density of the substantially manganese-free aluminum-based additive by physical blending with substantially oxygen-free recycled iron units, both to prevent parasitic reaction of aluminum with iron oxide and to avoid solid MgOxe2x80x94Al2O3 spinel and FeO/MnOxe2x80x94Al2O3 hercynite/galaxite spinel aluminate inclusions formation, deleterious to steel castability and quality.
These and other objects of this invention will be evident when viewed in light of the drawings, detailed description, and appended claims.